Various environmental and industrial chemicals can disturb male reproductive function. The objective of these studies is to enhance our understanding of that toxic potential, and to further elucidate the mechanism of action in chemicals found to be toxic. Chemicals which are active as chemosterilants in males, such as kepone and dibromochloropropane, are used in various test systems. In addition to mechanistic studies, chemicals of unknown activity, such as the phenoxy herbicides and TCDD, have also been studied. Endpoints of toxicity include the assessment of spermatogenesis, sperm morphology, hormone levels and specialized chromosomal or genetic assays. Studies have also involved the assessment of progeny survival and development of treated males. These studies are expected to yield valuable data on chemical toxicity in males, as well as improve the sensitivity and accuracy of future testing systems.